Nondestructive testing has proven to be an essential tool for assessing corrosion and erosion related damage in large equipment and structures. For long lengths of structure, such as pipes, tubes, steel cables, and plates, long range guided wave inspective has proved to be effective. Using long range guided wave techniques, a long length of structure can be inspected quickly and economically from a given probe location.
One example of a long range guided wave technique is an ultrasonic technique based on the use of Lamb waves. The guided waves are generated by using piezoelectric probes and mechanically coupled into the material under inspection. The Lamb waves propagate between two parallel surfaces and can be used to detect changes in wall thickness. This technique is described in U.S. Pat. No. 6,148,672, to Cawley, Alleyne and Chan, entitled “Inspection of Pipes,” assigned to Imperial College of Science, Technology of Medicine (London).
Another example of a long range guided wave technique uses magnetostrictive probes. These probes generate and detect mechanical waves in ferromagnetic material. Testing of a pipe involves two sets of an inductive coil encircling the pipe and a bias magnet. A time-varying magnetic field is applied to the pipe by the transmitting coil and this generates an elastic wave in the pipe due to the magnetostrictive effect. The waves propagate along the pipe in both directions. The receiving magnetostrictive probe uses the other encircling coil to detect changes in magnetic induction in the pipe due to the inverse magnetostrictive effect when the waves pass through. The system does not require a couplant or direct contact. This technique is described in U.S. Pat. No. 5,581,037, entitled “Nondestructive Evaluation of Pipes and Tubes Using Magnetostrictive Sensors,” to Kwun and Teller, assigned to Southwest Research Institute (San Antonio, Tex.).
Guided waves can exist in various modes, such as longitudinal, flexural, and torsional wave modes in pipe or tube, and symmetric and antisymmetric Lamb wave and shear horizontal wave modes in plate. In each wave mode, they can exist in different forms, with the forms referred to as “orders”. Guided waves are also dispersive, and therefore, their velocity of propagation varies with wave frequency.
To simplify analysis of the detected signals, the probes for long range guided waves are often controlled to generate a single wave mode of a fixed order that is propagated in a given direction, and to detect the same wave mode traveling in the opposite direction after reflection from a defect or a normal geometric feature such as a weld.
In practice, the control of wave mode and directionality is imperfect. As a result, extraneous wave modes are generated and detected, and extraneous waves may be propagated in directions other than the intended direction. These extraneous signals can be confused with the desired reflected signals, leading to erroneous data interpretation. Because of the complexities of guided waves, as well as of geometric features in structures, proper analysis of acquired data is time consuming and requires extensive knowledge of guided wave properties and interactions with geometric features in the structure.